The present invention relates to menstrual calculators and more particularly to a dial type calculator for determining periods of fertility and sterility in a menstrual cycle and relating such data to the days of two consecutive months over which the menstrual cycle usually occurs.
Manually operated calculators, both of the dial type and linear type, are well known in the art of menstrual calculators. Based on available statistical data, such calculators are used to predict the onset of a menstrual cycle that follows a current menstrual cycle and to indicate periods of fertility and sterility within the current cycle.
The prior art shows that the foregoing devices are based on statistical reports of medical studies which reveal the existence of a rhythmic pattern between ovulation and the onset of the next menstrual cycle. During the menstrual cycle, the period of fertility is related to ovulation which occurs with the rupture of a Graafian follicle and the attendant release of an ovum. This natural event forms the basis of the "rhythm system" which is a widely practiced method for family planning. There are other methods for family planning which are also widely practiced and include orally ingested steroids as well as known mechanical devices that are available for either the male or female member of a married couple. The effect of these devices provides a reliable means for birth control. Nevertheless, for many couples recourse to any method other than the rhythm method is unacceptable due to religious, aesthetic or psychological reasons. The rhythm method for many couples is therefore the only solution to family planning.
The basis of the rhythm system is founded upon assumptions derived from currently available medical data. Medical studies indicate that ovulation occurs fourteen to sixteen days prior to menstruation. The prior art tends to be in agreement in this respect. Having determined the date of ovulation merely by counting backwards, fourteen, fifteen or sixteen days from the onset of the next following menstrual cycle, other factors must then be taken into account in order to determine with reasonable certainty periods of fertility and sterility within the current menstrual cycle. However, there appears to be some divergence in the prior art as regards the factors which govern the length of the fertile period during any menstrual cycle.
There is general agreement that the duration of the fertile period and its location within the current menstrual cycle varies in accordance with the date of onset of the next following menstrual cycle. While ovulation and menstruation are parts of the same cycle, statistics have shown that the time of ovulation is more directly related to the onset of the following menstrual cycle than to the preceding cycle. Since the date of onset of the next cycle is never known, only an estimate can be used. This date therefore comprises a variable quantity which is best estimated by reference to the past history of a woman's menstrual periods.
Further variables occur which also affect the periods of fertility and sterility in the menstrual cycle. It has been stated that ovulation occurs between fourteen and sixteen days prior to the next menstrual cycle. It is apparent, therefore, that there are two variables here, the first being the prediction of the onset of the next menstrual cycle and the second being the selection of one of the three days during which ovulation is expected to occur.
Still other variables complicate the situation due to the life span of the male sperm which is estimated to be between two and three days and the life span of the female ovum which is estimated to be twenty-four hours. It is apparent that the life span of the male sperm extends the fertility period to approximately the nineteenth day before onset of the next menstrual cycle whereas the life span of the ovum brings the fertile period to eleven days prior to the onset of the next menstrual cycle. A safety factor is then usually added to the first and last days of the fertility cycle to take into account such effects as the menstrual period not starting at the beginning of the day. Accordingly, the fertile period may occur from eleven through twenty days prior to the onset of the next following menstrual cycle. It is thus apparent that a calculation to determine the fertile and sterile periods occurring during the menstrual cycle is at best difficult. Moreover, the calculation is further complicated by physchological factors which can affect initiation of ovulation.
Many prior art calculators have been developed in an effort to introduce variable compensation to account for the aforenoted variables. The end result, however, is often a complicated device having generally a multiplicity of scales or at best a reduced number of scales but with a large number of graduations on some of the scales used. The result is a calculator device that appears confusing to the eye and is therefore difficult to read. Moreover, such a device generally has a large number of dial graduations which makes it still more difficult to read, particularly if the device is small in size. Furthermore, since there are a number of variables that must be considered and such variables are affected by psychological factors that may vary from day to day, the accuracy of the results obtained may rightly be questioned; particularly so in light of the complexity of the device and the difficulties experienced in using it.
A perfectly normal menstrual cycle has a duration of twenty-eight days. However, the range of a normal menstrual cycle may vary from twenty-six to thirty days. Some of the prior art literature, however, specifies that most menstrual cycles are between twenty-two and thirty-four days in length and vary from month to month by from two to eight days. It is apparent therefore that the menstrual cycles of most women vary continually and that they are predictable between points of maximum and minimum duration. Statistical studies have shown that for a woman who has menstrual cycles ranging from twenty-six to thirty-one days in length, more of her cycles will be twenty-eight and twenty-nine days rather than twenty-six, twenty-seven, thirty or thirty-one days.
The foregoing suggests that a statistically average woman could reliably use a fertility calculator that is more simple in construction than similar calculators in the prior art by virtue of the absence of compensating scales needed to adjust the period of fertility because of the variables. Since the normal menstrual cycle variation may be only about two days, it should be possible to provide a more simple calculator having good reliability for the woman whose cycles range between twenty-eight and twenty-nine days.